Study finds context-dependent behavior in antlions after ‘playing dead’

A study by scientists at the University of Bristol reveals what animals do after feigning death to avoid being killed by a predator and the context of this behaviour. The work was published in PLoS ONE.

Many animals, as a last defense, become immobile after being contacted by a predator.

This behavior is so common that it is recognized in expressions such as “possuming.” It is even said to occur in humans in extreme circumstances.

In previous studies by the same team on antlion larvae, scientists noticed that the larvae became immobile after being handled individually.

At some point, the larvae need to be weighed, which can be very difficult with such small insects. If they move around on the scale, determining their mass can be difficult.

However, when the antlion larvae were placed, very gently, on the pan of a balance, they remained completely still for long enough for their weight to be accurately recorded.

Emeritus Professor Nigel Franks from the University of Bristol’s School of Biological Sciences, who led the study, said: “We chose to study this so-called ‘death-faking’ behaviour and found that the length of time antlions remain stationary is completely unpredictable for any given individual.

“This is confirmed by the observation of post-contact immobility times in a large number of antlions. These data show an exponential distribution. Thus, just as for radioactive atoms, the moment when an individual changes state is unpredictable, but the population pattern is perfectly predictable.”

The study shows that the antlions’ hiding-in-plain-sight behavior is likely adaptive, because a predator that picked up and dropped an antlion larva could not know how long to wait for its potential victim to move again and become recognizable prey. Indeed, one of the antlions recorded remained completely still for more than an hour.

Although it is impossible to predict when a motionless antlion will come back to life, this does not mean that the predator will necessarily have left the area to seek alternative prey.

The team then asked what the animals did after playing dead. In the new study, they showed that what the antlions do depends on the situation they find themselves in.

Antlion larvae are burrowing animals that may seek shelter by immersing themselves in the soft substrate where they normally build their burrows. But it is quite possible for a predator to drop an antlion onto a hard substrate that would not allow it to escape by borrowing.

Using sophisticated automated video tracking of the intermittent locomotion of individual antlions on different substrates, the researchers found that what an antlion does after ending its period of immobility depends on the escape strategies available.

Professor Franks added: “Our study may well be the first to determine what animals do after they play dead, and we show that what they do depends on the context. It’s a trade-off. So our work opens up the field of studying life after death in a wide range of animals that pretend to die, thanatosis or what we prefer to call post-contact immobility.”